Whenever Cyclops Gladiator attacks, you may have it deal damage equal to its power to target creature defending player controls. If you do, that creature deals damage equal to its power to Cyclops Gladiator.
Cyclops Gladiator's ability triggers and resolves during the declare attackers step, before blockers are declared. If it survives, it continues to attack (presumably with some damage marked on it) and may be blocked.
If the targeted creature leaves the battlefield (or otherwise becomes an illegal target) before the ability resolves, the ability doesn't resolve. Cyclops Gladiator isn't dealt damage.
On the other hand, if Cyclops Gladiator leaves the battlefield before the ability resolves, the ability continues to resolve. Cyclops Gladiator deals damage to the targeted creature equal to the power it had as it last existed on the battlefield.